This invention relates generally to methods and apparatus for administering oxygen and other respirating gases to a patient.
More particularly, this invention relates to methods and means for administering oxygen or other gases to a patient on an intermittent, respiration controlled, basis.
It has become common medical practice to treat patients suffering from advanced stages of chronic obstructive pulmonary diseases by administration of oxygen. Such pulmonary diseases, including chronic bronchitis, emphysema and severe asthma, are one of the fastest rising causes of death in the United States affecting probably more than ten million people. It is estimated that more than 500,000 people in the United States either are routinely receiving oxygen therapy or could benefit from it. Much of this rather expensive treatment is funded by Medicare.
The devices which commonly have been used to deliver oxygen to a patient meter the oxygen flow at a fixed rate and deliver a constant stream of oxygen to the patient. Oxygen is received by the patient either through a mask which is placed over the nose and mouth or through a cannula which terminates in nares inserted into the patient's nostrils.
It has long been recognized that providing a constant flow of oxygen to a patient results in the waste of a substantial proportion of the oxygen supply. The normal breathing cycle consists of an inhalation, an exhalation longer in duration than the inhalation, and then a pause before the next cycle. Oxygen supplied to a patient during the exhalation and pause phases of the respiratory cycle is totally wasted. Devices have been developed to conserve oxygen by regulating the oxygen flow, turning it on and off, in response to the respiratory cycle. Typical of such devices are those of Myers, U.S. Pat. No. 4,054,133, and Mon, U.S. Pat. No. 4,381,002. Each of these patents disclose devices which sense inhalation and exhalation pressures in the nasal cavity of a patient and converts those sensed pressure differentials to signals which control the flow of oxygen to a patient. Typically, oxygen flow is started upon the sensing of a negative pressure relative to atmospheric indicating the start of an inspiration period. Oxygen flow is then stopped at a second signal produced by the sensing of a positive pressure relative to atmospheric indicating the start of the expiration period.
More recently, Dr. Gerald Durkan clinically observed that only the oxygen supplied during the initial part of an inspiration period was efficiently absorbed by a patient. It is that first inspired portion of oxygen which reaches the alveoli while oxygen supplied during the latter part of an inspiration period remains in non-absorbing areas such as the pharynx, trachea and bronchial tubes. Durkin concluded that supplying oxygen at a high rate, beginning at the start of inspiration but lasting only for a small portion of the inspiration period offered economic and physiologic advantages over those prior techniques which supplied oxygen during the entire inspiration period.
As a result of his observations, Durkan developed a respirator system known as the Demand Oxygen Controller which is disclosed in U.S. Pat. No. 4,457,303. That respirator system uses a fluidic laminar proportional amplifier to sense the start of an inspiration period. Oxygen flow to a patient is immediately started in response to the sensed inspiration. Timing means, also started in response to the sensed inspiration, stop the oxygen flow after a preset period of time which is shorter than is the inspiration period. As a result, oxygen is supplied to a patient only during the effective, early stages of an inspiration resulting in an oxygen savings of as much as 70% as compared to a continuous flow administration.
All of these prior art techniques have one property in common. All determine or control the volume of oxygen (or other respirating gas) delivered to the patient over a respiratory cycle by controlling both the rate at which oxygen is allowed to flow and the time or duration of oxygen flow for each respiratory cycle. Both Myers and Mon teach the starting of oxygen flow upon sensing the beginning of an inspiration and the stopping of oxygen flow upon the sensing of an expiration. Durkin teaches the starting of oxygen flow upon sensing the beginning of an inspiration and stops oxygen flow at the end of a time period which is independent from, and shorter than, the inspiration period.
Those prior art devices which connect a rate metered supply of oxygen to a cannula for a predetermined time to effect the desired dose delivery all have the disadvantage that both the rate and duration of flow must be precisely controlled if the dose is to be accurately measured and dispensed. Because of the small quantities of oxygen required per dose (typically about 33 cc measured at standard temperature and pressure), it is difficult and expensive on a production basis to provide for the degree of accuracy of flow rate and of timing required to ensure a safe dose efficiently delivered for each breath. An improved and simplified way to dispense an oxygen dose in synchronization with a patient's respiratory cycle provides clear advantages in this art.